1. Field of Invention
The present invention relates to a method for recovering zinc from dust, and more particularly to a method for recovering zinc from dust containing iron as a main component.
2. Description of the Related Arts
Since a dust recovered in steel works has a high iron content, the dust is efficiently utilized as an iron source. However, some of the dust contains zinc. When such zinc containing dust is charged into a blast furnace, the normal operation of the blast furnace may be hindered. Accordingly, a dust containing zinc at an excessive amount can not be used as a raw material for the blast furnace.
For example, a blast furnace dust collected by a dust catcher of the blast furnace contains approximately 0.1 to 3 wt.% of zinc coming from iron ore, and a converter dust collected by a dust collector contains approximately 0.1 to 3 wt.% of zinc coming from galvanized steel sheets or the like which were charged to the converter as a scrap. When these dusts are charged to the blast furnace, the zinc components are reduced to metallic zinc, and the metallic zinc is melted to vaporize because it has a low boiling point. The melting point of metallic zinc is 420.degree. C. and the boiling point is 920.degree. C.
The vapor of metallic zinc ascends in the blast furnace along with the reducing gas, and it is cooled while ascending. A part of the metallic zinc vapor is discharged to outside of the blast furnace with the gas (reducing gas) of the blast furnace. However, the rest of the metallic zinc vapor adheres to the furnace wall surface in a form of liquid or solid before reaching the top of the furnace. If the metallic zinc adheres to the furnace wall surface and if it grows on the wall surface, then the gas permeability within the furnace is decreased and the operating condition of the furnace becomes abnormal. To prevent the furnace operation from such a bad state, currently the zinc content of the burden being charging into the blast furnace is controlled not to exceed 0.2 kg per ton of hot metal. Accordingly, the application of the dust containing zinc is performed after removing zinc.
A conventional method for removing zinc from a zinc containing dust and for recovering it is disclosed in unexamined Japanese patent publication No. 144437/1983. FIG. 3 illustrates the disclosed method. The reference number 40 denotes a hopper holding zinc containing dust, 44 denotes a mixer car holding a high temperature hot metal 62, and 48 denotes a wet separator having a water tank for dust collecting. The reference number 41 denotes an oxygen blowing pipe for transporting dust, 42 denotes a dust transfer pipe, 43 denotes a top blowen lance, 45 denotes a hood, 46 denotes a duct, and 47 denotes a fan.
The dust in the dust hopper 40 is transferred by oxygen blown through the oxygen blowing pipe 41, and is injected into the hot metal 62 in the mixer car via the top blowen lance 43. The blown dust is heated by the hot metal 62 and is reduced by carbon in the hot metal. The iron oxide in the dust melts into the hot metal, and the zinc oxide in the dust is vaporized and is sucked along with the powdered dust in the mixer car 44. The vaporized zinc is sent to the wet separator 48 and are then collected in water.
However, the above-described method has following problems. Since powdered dust is charged into the hot metal 62, bubbles generated in the hot metal contain dust. When the ascended bubbles break at the surface of the hot metal, the dust in the bubbles is carried over with the sucked flue gas or suspended at the surface of the hot metal. The suspended dust is difficult to melt into the hot metal, and a part of it is carried over. As a result, the wet separator 48 accepts the dust which has been blown into the hot metal and the dust is left in a state of being neither reduced nor vaporized. Consequently, the collected dust shows a very low zinc content and very high iron content, and the recovery efficiency of iron to hot metal becomes low.
Since the blown gas includes oxygen, fine particles of iron oxide which are newly generated by the blown oxygen are also carried over, and these particles are also collected by the wet separator 48. Accordingly, the zinc content in the collected dust is further lowered, and the zinc content may become lower than that in the supplied dust depending on the operating condition. Furthermore, the generation of the new fine particles of iron oxide degrades the recovery rate of iron into the hot metal.
When oxygen is included in the blown gas, the atmosphere in the container holding the hot metal likely becomes an oxidizing one. Therefore, the dust floating on the hot metal surface is difficult to be reduced. As a result, the quantity of zinc being vaporized is decreased, and the recovery rate of zinc is decreased. Also the iron oxide is likely discharged along with slag, and the iron recovery rate into the hot metal is lowered.